The field of the present invention is defrosting controls as used in heat pumps.
In utilizing heat pumps for heating, the heat pump outdoor coil must, to take up heat, be at a temperature below the outdoor ambient air temperature. In ordinary operation, the temperature of the coil may be below the outdoor ambient air dew point, causing water vapor to condense on the surface of the coil; if the surface temperature of the coil is below 0.degree. C., frost may appear on the coil. Rainwater may likewise collect on the coil and freeze. To remove the frost, a defrost cycle is initiated, which usually includes de-energizing the outdoor coil fan and reversing the heat pump four-way reversing valve; this causes heat to be transferred from indoor coil to the outdoor coil. As the outdoor coil is heated, the frost melts and the water drains away or evaporates.
In the prior art, various systems have been utilized to control defrosting operation. The simplest systems have a clock which iniiates defrosting at selected time intervals. Other systems, such as disclosed in U.S. Pat. No. 3,466,888 to Kyle, rely on the temperature difference between the outdoor air and the outdoor coil; a difference exceeding a selected level, such as 12.degree. F., indicates frosting. Another system, disclosed in U.S. Pat. No. 4,104,888 to Reedy, et al., initiates defrosting when the compressor current exceeds a selected level; the abnormally high current indicates a frosted outdoor coil. More complicated systems utilize a lamp and photocell to directly detect ice build-up between the coils, or very sensitive pressure transducers to detect pressure drop through the coil resulting from frosting, as shown in U.S. Pat. No. 3,377,817 to Petranek. Each of these systems requires sensitive adjustments in order to operate properly.